Interlayer charge transfer in supported and suspended MoS2/Graphene/MoS2 vertical heterostructures

In this letter, we report on the optical and structural properties of supported and suspended MoS2/Graphene/MoS2 vertical heterostructures using Raman and photoluminescence (PL) spectroscopies. Vertical heterostructures (VH) are formed by multiple wet transfers on micro-sized holes in SiO2/Si substrates, resulting in VH with different configurations. The strong interlayer coupling is confirmed by Raman spectroscopy. Additionally, we observe an enhancement of the PL emission in the three-layer VH (either support or suspended) compared with bare MoS2 or MoS2/Graphene. This suggests the formation of a spatial type-II band alignment assisted by the graphene layer and thus, the operation of the VH as a n++/metal/n junction.


Introduction
Semiconductor heterostructure junctions are the basic building block of modern solid-state devices since they enable the engineering of different degrees of freedom such as bandgap, conductivity, and refractive index, among others. Epitaxial growth of thin-film heterostructures is routinely achieved by molecular beam epitaxy (MBE) [1][2][3] or metal-organic chemical vapor deposition (MOCVD) [4,5]. Even though these techniques offer high-crystallinity and precise control of the thin-film structure during the growth, one of the limiting factors is the choice of the materials since the lattice mismatch has to be near zero to avoid the formation of dislocations [6]. In contrast, due to the absence of dangling bonds and Van der Waals forces between layers [7] the vertical junction of two-dimensional (2D) materials offers an essentially unlimited number of possibilities regarding the choice of the materials and their stacking configuration, including the relative orientation angle between layers which can dramatically change their properties [8,9]. This opens a new gamut of opportunities to explore new physical phenomena and design novel ultra-thin devices.
It has been shown by first-principles calculations that the insertion of graphene between two different transition metal dichalcogenides (TMDs) is thermally stable and does not modify their intrinsic band structure. In addition, the resulting heterostructure configuration considerably enhances the interlayer hopping of photogenerated carriers between the conduction (CB) and valence band (VB) of the materials [32]. Experimentally, photoluminescence (PL) and Raman spectroscopy have been demonstrated to be reliable non-invasive techniques to monitor the interlayer charge transfer in VHs since the optical phonons and the radiative and non-radiative recombination paths are highly sensitive to charge doping [33][34][35].The novelty of our work stems from using the same TMDs (MoS 2 monolayer) separated by a graphene layer to demonstrate that a similar charge transfer mechanism can also be achieved using the same semiconductor layered materials.
In this work, we study the optical properties of MoS 2 /Gr/MoS 2 VH supported on SiO 2 and suspended on micro-sized holes by means of Raman and temperature-dependent microphotoluminescence (PL) spectroscopies. The technique used to fabricate the VHs results in different stacking configurations (MoS 2 , MoS 2 /Gr, and MoS 2 /Gr/MoS 2 either supported or suspended) which allows to compare their properties. The position and width of the MoS 2 and graphene Raman bands reveal strong interlayer coupling between the layers. The PL intensity quenching observed in MoS 2 /Gr VH is attributed to the charge transfer mechanism from MoS 2 towards graphene. In stark contrast, the PL intensity of the MoS 2 /Gr/MoS 2 VH, either suspended or supported, is significantly larger than the rest of the above-mentioned configurations, which is indicative of different electronic properties. We propose that the observed PL enhancement is originated from the formation of a spatial type-II band alignment induced by the insertion of the graphene layer between the MoS 2 monolayers.

Sample preparation
Growth of MoS 2 : MoS 2 crystals were synthesized using an atmospheric pressure chemical vapor deposition (APCVD) system on silicon dioxide (SiO 2 ) substrates (S1a). The substrates were immersed and cleaned using an ultrasonic cleaner in acetone and isopropanol baths. The substrate is then placed on top of an alumina boat containing molybdenum dioxide (MoO 2 Sigma Aldrich 99%) located at the center of the chamber, a second boat containing sulfur (99.5% Alfa Aesar) was loaded 15 cm away from the furnace center. MoS 2 single layers were principally obtained by raising the temperature furnace at 750 at a 50˚C /min ramp using 100 sccm argon flux as a carrier gas and immediately cooling the system after reaching 750˚C. Optical and atomic force microscope images confirm the presence of monolayers (S1B and S1C Fig).
Growth of graphene: Large-area graphene layers were grown on 25 μm thick Cu foils (Sigma-Aldrich, 99.98%) as substrate in a low-pressure CVD chamber. The cleaning of the substrates consisted of different baths with ac. nitric acid (5.4% by volume) 30 seconds, acetic acid (4% by volume) 60 seconds, acetone, isopropanol and drying with a flow of nitrogen gas. The Cu foil was annealed at 1010˚C for 30 min in 2 sccm H 2 , after this time the H 2 was turned off and a CH 4 flow of 20 sccm was introduced into the chamber for 30 min to promote growth, to cool the chamber 50 sccm of Ar.

Microphotoluminescence and Raman characterization
Raman and PL spectra were measured at RT using a micro-Raman spectrometer (LabRAM HR Evolution, HORIBA), and a laser excitation wavelength of 488 nm with a laser power of 0.5 mW was used on the sample, both Raman and PL spectra were measured with a 600 gr/ mm -1 grating. The laser radiation was focused onto the substrate surface with a spot of 1 μm diameter using a 100X microscope objective with 0.4 numerical aperture. For measurements at low temperature, the sample was placed in a closed-cycle helium cooled cryostat and examined in micro-PL configuration. A 515 nm diode laser focused with a 50x microscope objective was used to excite PL The emission is analyzed with a spectrometer (HORIBA iHR550) equipped with a CCD camera.

Results and discussion
Micrometer-sized holes on the SiO 2 /Si substrates were defined using optical lithography. The sample was selectively wet-etched using HF to form holes of approximately 1 μm depth. Subsequently, the MoS 2 /Gr/MoS 2 heterostructure was fabricated by multiple transferring of CVD MoS 2 crystals and graphene using the cellulose acetate-assisted method [36]. Details about the growth of MoS 2 flakes and graphene layers can be found in the methods section. Finally, a thermal annealing treatment at 300˚C under argon atmosphere was carried out for two hours to improve the coupling between each material. Fig 1A and 1B show the schematic and optical images of the samples after the artificial heterostructure assembly. The resulting sample consists of different 2D structures, including MoS 2 , MoS 2 /Gr, and MoS 2 /Gr/MoS 2 either suspended on the hole or supported on the SiO 2 substrate (A step by step schematic and optical images are shown in S2 and S3 Figs). For simplicity in the following, we will label bilayer and trilayer to make reference to MoS 2 /Gr and MoS 2 /Gr/MoS 2 VHs repectively. . The A 1g peak remains around the same position for all the structures except for the suspended bilayer where a strong blue-shift is noticed. The blue-shift can be ascribed to a phonon renormalization due a different carrier concentration compared with other VHs revealing the strong interlayer coupling between the two materials [35]. Interestingly, the position of the E 2g and A 1g Raman bands measured on the suspended and supported trilayer are very similar to bare MoS 2 , and the difference in wavenumber (18 cm -1 ) indicates that the top and bottom MoS 2 crystals be considered as monolayers. Finally, considering wavenumber value and the ratio of the G and 2D Raman bands, the Lorentzian fitting of the 2D peak, and the wavelength excitation (488nm), we confirm that the thickness of the trilayer VH is indeed three monolayers (S5 Fig and S1 Table).
Density functional theory calculations [38] and differential reflectance spectroscopy [39] have demonstrated that in a bilayer VH, the A 1g blueshift can be attributed to the modification

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of the charge concentration in the MoS 2 due to charge transfer from MoS 2 to the graphene. Note, that this blue-shift is larger in the suspended bilayer than in the supported ones, where charges on the substrate may screen these effects (see curves II and III in Fig 2(A)). Further evidence of the charge transfer in the bilayer from MoS 2 to Gr is the 9 cm-1 blue-shift of the 2D Raman mode of graphene in the VHs (curve IV in Fig 2(B)).
Having established the existence of charge transfer from MoS 2 to Gr, we investigate and compare the PL emission at RT of the three suspended VHs (Fig 3A). For comparison purposes, we measured the PL spectra of the isolated MoS 2 flake suspended over a hole in a separate sample (S6 Fig). The three samples present two emission features, between 1.85 and 1.88 eV, and another at 2.05 eV, commonly attributed to, respectively, the so-called A and B excitons associated to transitions the direct transition channels [40] from the highest spin-split valence band to the lowest conduction band at the K point of the Brillouin zone. One can make several interesting observations from the PL spectra. First, the signal of the exciton A in

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the bare MoS 2 sample peaks around 1.88 eV with a full half-maximum width (FWHM) of 80 meV. In the case of the bilayer, the PL peaks at 1.87 eV and has a similar FWHM than bare MoS 2 . In contrast, there is a notable broadening of the PL spectrum of the trilayer. From the deconvolution of this luminescence spectrum (Figs S7 and 5A), we found a third contribution to the PL signal assigned to the negatively charged exciton or trion X -. Note that there is as well significant variation in the PL signal intensity, which will be discussed later.
The emergence of the trion signal in the trilayer indicates a larger carrier concentration in the MoS 2 than in the other two configurations measured. To explain this feature, we propose an analogous mechanism as that described in Ref 32., where two TMDs monolayers separated by graphene form a spatial type-II band-alignment. In our case, the trilayer forms an n++ semiconductor/graphene/n semiconductor junction, where the photo-excited of the top MoS 2 monolayer are quickly transferred to the graphene and immediately diffuse towards the conduction band of the bottom MoS 2 monolayer, promoting the formation of the trions (Fig 3B). Further evidence of this is found in the Raman spectra from the supported and suspended trilayer VHs, where the shift of the 2D band related to doping of graphene is 6 cm -1 , which was smaller than for the bilayer. The smaller blue-shift indicates a smaller carrier concentration in the graphene, demonstrating that acts as a charge mediator between both MoS 2 layers.
We now turn our attention to the PL intensity emission recorded VH configurations ( Fig  3C). We observe that while the signal intensity of the bilayer supported on SiO 2 decreases in comparison with bare MoS 2 on SiO 2 , it dramatically diminishes for the suspended bilayer heterostructure. As pointed out in the discussion about Raman characterization, the PL quenching is attributed to the interlayer charge transfer that modifies the recombination paths of the photogenerated species A and B excitons and the charged exciton or trion [38]. In the case of the suspended bilayer, the PL intensity further decreases by the reduction of amplification by optical interference due to the lack of the SiO 2 spacer [41]. The effect of the substrate can also be observed in S8 Fig In contrast to the suspended bilayer, the suspended trilayer VH does show an important broadening, further supporting our assertions regarding that the formation of trions in this VH is related to graphene rather than to the free charges on SiO 2 .
Moreover, the PL measured on the trilayer displays a similar or even slightly larger intensity (Fig 3C) than bare MoS 2 regardless of the VH being suspended or supported on SiO 2 . This can be ascribed to the internal loop created by the formation of the spatial type II band alignment, which would lead high emission efficiency due to the short times of the e-h recombination process in the stacking trilayer, explaining the high PL emission on the supported or suspended regions.
To further investigate the role of the graphene layer in the trilayer, we measured the PL of MoS 2 bilayers under the same conditions as the trilayer VH. Fig 4 shows that the PL emission of the bilayer presents a significant quenching compared to bare monolayer MoS 2 on SiO 2 either when suspended or supported on SiO 2 . This is well-known to be related to the emergence of the indirect bandgap recombination path [43], showing the importance of the graphene layer to form the spatial type II alignment while preserving the direct character of the transition. Interestingly, this point towards the possibility to use the graphene layer to control the charge density on the MoS 2 layers by applying an external gate bias to control the Fermi energy level.
As mentioned before, the PL spectra of the trilayer presents broadening. To identify and compare the different PL features in all the samples, we performed a Lorentzian fitting of the PL spectra, revealing three excitonic resonances peaked around 2.05, 1.87, and 1.83 eV ( Fig  5A). In one hand, while there is significant emission due to the trion formation in bare MoS 2 on SiO 2 , when graphene is stacked on MoS 2 , exciton A becomes the predominant contribution (top curves of Figs 5A and S9A). On the other hand, the integrated intensity area of the trion feature ( S9B Fig) increases with the excitation intensity only for the suspended trilayer, suggesting that part of the electrons recombine at the surface of the substrate when the trilayer is laying directly on the SiO 2, limiting the trion recombination path.
Finally, we performed micro-PL in the temperature range from 15K-200K on the trilayer suspended region. It is noteworthy that no PL signals related to bound excitons were observed, indicating low defect densities in both MoS 2 CVD monolayers forming the VH. The

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luminescence of both excitonic resonances, A exciton and trion, progressively blueshift with decreasing the temperature from 200K to 15K. By fitting the spectra, we noted that the normalized intensity of the exciton A decreases while the trion intensity increases at lower temperatures (Fig 5C). In addition, it is possible to reproduce the variation of the peak position of each PL feature applying the Varshni expression [44]: where E (0) is the energy peak position of the trion or exciton A at 0K and α and β the fitting parameters of the material. Using the data shown in Fig 5D we found the following values: for the exciton A α = 0.33meV/K and β = 159K and for the trion α = 0.37 meV/K and β = 143K. These results along with the obtained values for the fitting parameters of the Varshni equation, are in good agreement with low temperature PL experiments performed on single MoS 2 [45,46] indicating that the recombination process of the trilayer is close to an individual MoS 2 layer.

Conclusions
In summary, we have studied the optical properties of trilayer (MoS 2 /Gr/MoS 2 ) heterostructures suspended and supported on SiO 2 substrates. The photoluminescence emission intensity from the trilayer structures indicates the formation of an n++ semiconductor/graphene/n semiconductor, resulting in a spatial type II band alignment where graphene acts as a charge mediator between the MoS 2 monolayers. The importance of our results derives from the fact

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that a graphene layer laying between two monolayers of the same material forms a type II band alignment, which is normally achieved using different TMDs. Moreover, the graphene layer can be electrically back-gated to move upward or downwards the Fermi level and control the carrier density in the MoS 2 monolayers, either to enhance or decrease the PL emission. Therefore, we propose that this configuration could be used as an electrical tunable interface to develop novel optoelectronic and electronic devices.